Concentric conductor transmission line



April 21 1942. P, H, SMITH 2,280,200

CONCENTRIC CONDUCTOR TRANSMISSION LINE Filed April 16, 1940 FIQJ .w wz!/// W INVENTOR hi SMITH A T TQRNE V UNITED STATE Patented Apr. 21,1942 CONCENTRIC conngg'gon TRANSMISSION PhillipJI. Smith, Denville, N.J., assignor to Bell Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application April 16, 1940,Serial No. 329,862

3 Claims. (Cl. 174-28) This invention relates to an improvement inconcentric conductor transmission lines.

One form of concentric conductor line known heretofore comprises atubular inner conductor disposed in a tubular outer conductor and spacedconcentrically therefrom by insulating rings, or the like, mounted onthe periphery of the inner conductor at intervals therealong. In sucharrangements the insulating rings are held in pl" ce on the innerconductor in one of the following ways: (a) friction, (b) crimping theinner conductor on both sides of the insulating ring, soldering metalrings on the inner conductor on both sides of the insulating ring, and(d) springing resilient wires on the inner conductor on both sides ofeach insulator.

Accordingly, these insulating rings create gaps between themselves andthe inner conductor. along a radial extending from the center of theline, and, in addition, introduce between the in ner and outerconductors a relatively large mass of material.which has ahighdielectric constant compared to air or gas surrounding it and which,in addition to increasing the dielectric losses that are proportional tothe mass of this material, alters the distribution of the electrostaticfield and consequently the distribution of voltage gradient on the outersurface of the inner conductor. This distribution would be uniform for aline assumed to have a uniform air or gas di- 5 electric interveningexclusively between the inner and outer conductors, or a line providedwith insulators having a dielectric constant equivalent to that of-thesurrounding air or gas, if such were possible. However, since a solidmaterial having a dielectric constant equivalent to that of air or gasis physically non-existent, the nearest aptially less than those thatwould otherwise be required to produce corona and flash-over on aconcentric line of the same cross-sectional dimensions and having air orgas dielectric intervening exclusively between the inner and outerconductors.

Also it may happen that either in slipping the outer conductor onto theinner conductor or in handling an assembled concentric conductor line inthe field, the insulating rings would be subjected to longitudinaldisplacement on the inner conductor. Such movement tends to change theconcentric relation between the inner and outer conductors, therebycausing further variation in the distribution of the voltage gradient onthe outer surface of the inner conductor.

Concentric conductor lines may be used in the transmission of broad bandsignaling waves in the manner disclosed in the patent of L. Espenschledet al. No. 1,835,031 issued December 8, 1931.

According to this invention there is provided a concentric conductorlineembodying pin-type insulators in which the flash-over factor of suchinsulators is higher than that of the line per se.

One object of the invention is to provide a concentric conductor systemwhich, for a given crosssection and conductor diameter ratio, allows theeflicient transmission of maximum signaling voltages, substantiallyequivalent to those that would be transmitted by a concentric conductorsystem of the same dimensions but embodying ideal insulatingarrangements.

Another object is to provide highly eflicient insulation for thetransmission of signaling currents embracing a wide band of frequencies.

Another object is to provide a substantial reduction in the cost ofmanufacture.

A further object is to provide in effect a min mum disturbance of thedielectric intervening between the inner and outer conductors. 4

- Another object is to improve the voltage flashover factor of theinsulators.

Another object is to reduce the voltage gradient on the surface of theinner conductor, in the vicinity of the points of engagement between theinner conductor and the insulators.

A further object is to provide for wide distribution of theelectrostatic lines of force in the vicinity of the points of engagementbetween the inner conductor and the insulators.

In a preferred embodiment of the invention, a concentric conductor linecomprises inner and outer conductors of which the inner conductor isprovided with a plurality of pairs of diametrically aligned openingsspaced therealong in such manner that adjacent aligned pairs areangularly disposed relative to each other. The openings of each alignedpair have different diameters and are arranged such that. the opening oflarger diameter is formed with an internally projecting perimeter. Ineach pair of aligned 'openings is disposed one end of an elongatedinsulator whose opposite end engages the inner surface of the outerconductor. This arrangement assures concentric spacing of the innerconductor within the outer conductor and at the same time improves theflash-over factor of the line in the vicinity of the individualinsulators as a relatively low potential gradient is' provided in thevicinity of the point of engagement between the internally projectingperimeter and the elongated insulator disposed therein.

This inventionwill be readily understood from i the followingdescription taken together with the accompanying drawing in which:

Fig. 1 is'a sectional view of a concentric conductor line illustrating apreferred form of .the

invention;

14. Also, preferably each opening I3 is angularly disposed 120 degreesrelative to adjacent openings l3 while each opening I4 is similarlydisposed relative to adjacent openings l4 as shown r in Fig. 2. Thelarger opening |3 of each aligned 1 pair is formed with an internallyprojecting perimeter l5 for a purpose that will be presently explained.It is understood that adjacent pairs of aligned openings l3 and I4 maybe disposed relative to each other in any suitable angular relation inaddition to the manner mentioned above.

An electrical insulator of an elongated type I having a rounded end 2|and formed with alongitudinal projection 22 on the opposite end is posi-'"tioned in each pair of aligned openings l3 and H such that theprojection 22 is disposed in the opening I4 and the rounded end 2|engages the inner surface of the outer conductor as shown in Figs. land2.

Material of which insulators 20 are made has a dielectric constant whichis high compared to that of the surrounding medium causing nearbyelectrostatic lines of force to pass through the insulators 20 inpreference to the more direct radial path to ground through thesurrounding medium. Due to the skin effect at high frequencies, however,or the tendency for current to crowd to the outermost edges and surfacesof I the inner conductor, a concentration of lines of force inthevicinity of the point of engagement between the inner conductor l2and the individ ual insulators 20, which would otherwise exist, isavoided by means of the internally projecting perimeter 5. Hence, therewill be a relatively small amount of current flowing along the innerconductor at this .point. In other words, the large surface. areaprovided by the inwardly projecting perimeter l5 results in a relativelywide distribution of the electrostatic lines of force.

- This eliminates a high voltage gradient in the vicinity of the pointsof contact between the inthe insulators.

ner conductor l2 and individual insulators 20 thereby reducing thetendency of flash-over alon the surface of the individual insulators 20.Such distribution of the electrostatic lines of force is illustrated invFig. 3. Thus, the inwardly projecting perimeters I5 serve to control theflashover factor of the individual insulators 20 which factor may bemade higher, if desired, than the flash-over factor of the concentricconductor line' per se. The radius of the perimeter I! determines, for agiven insulator and conductor diameter, whether the flash-over factor ofthe individual insulators 20 will be greater or less than that of theconcentric conductor line. A suiiiciently large radius is usually chosenfor the perimeters l5 so that the flash-over factor of the individualinsulators 20 will be higher than the flash-over factor of theconcentric conductor line.

Fig. 4 is generally similar to Fig. land illustrates an insulator 40comprising a cylindrical portion 4| of one diameter and a cylindricalportion 42 of a smaller diameter. The portion 42 is disposed in a pairof diametrically aligned openings 43 and 44 provided in the innerconductor. Shoulder 45 formed by the junction of the portions 4| and 42'engages the periphery of the inner conductor in a cup-shaped portion 46in which the opening 43 is embodied. Preferably, the shoulder 45 assumesa curvilinear shape that lends itself in a ready fit to the curvature ofthe outer surface of the inner conductor.

Fig. 5 is similar to Fig. 1 except that both diametrically alignedopenings are formed with internally projecting perimeters and a unitaryinsulator 50 extends therethrough.

In the above arrangements, it is obvious that the insulators areprecluded from longitudinal displacement on the inner conductor duringthe operations of (a) slipping the outer conductor onto the innerconductor, and (b) in handling an assembled concentric conductor -linein the field. Consequently, the concentric relation between the innerconductor and outer conductor is retained at all points along the line.

In addition, pin-type insulators with their minimum mass of highdielectric material intervening .between the inner and outer conductorstend to cause minimum dielectric losses and also minimum disturbances inthe distribution of the potential gradient on the outer surface of theinner conductor. This obviates the production of relatively steepvoltage gradients on the outer surface of the inner conductor in thevicinity of Consequently, signaling voltages that are substantiallyequivalent in magnitude to voltages allowed by a line of the samedimensions having air or gas dielectric intervening exclusively betweenthe inner and outer conductors, or an air orgas dielectric line of thesame dimensions provided with ideal insulators for maintainingconcentric spacing therebetween, may be applied to the line withoutcausing the production of corona or flash-over.

Furthermore, it is evident that a pin-type insulating arrangement lendsitself to the facile assembly of a concentric conductor line or theexpeditious replacement of defective insulators. In the latter event, adefective insulator can be removed and replaced without disturbing theother insulators, which is not so in the case of ring-type insulators asall insulators leading to the defective one must be removed before thelatter can be replaced. In addition, the inwardv1y projecting perimetersobviate the need of ac- I curately fitting the insulators as the extentof modifications is defined in the appended claims.

What is claimed is:

1. A concentric conductor transmission line comprising tubular inner andouter conductors, said inner conductor having a plurality of pairs ofdiametrically aligned openings, each of said pairs comprising openingshaving different diameters, the opening of larger diameter beingprovided with a perimeter depressed below the outer surface of the innerconductor, and insulators to maintain concentric spacing between saidconductors, each insulator comprising two portions of differentdiameters and a shoulder formed at the junction of said two portions,each insulator being disposed in one of said pairs of aligned openingssuch that the portion of the smaller diameter is positioned in theopening of smaller diameter and is'terminated on one end substantiallyat the outer surface of the inner conductor with the shoulder inengagement with a portion of the inner surface of the inner conductor inthe vicinity of the opening of the smaller diameter, and such that theportion ofthe larger diameter is positioned in the opening of largerdiameter to engage the depressed perimeter thereof and extend from theshoulder to the inner surface of the outer conductor.

2. A concentric conductor transmission line comprising tubular inner andouter conductors, said inner conductor having a plurality of pairs ofdiametrically aligned openings, one opening of each of said alignedpairs having its perimeter depressed below the outer surface of the in--ner conductor, and insulators to maintain concentric spacing betweensaid conductors, each insulator comprising two portions of differentdiameters and a shoulder formed at the junction of said two portions,each insulator being positioned such that the portion of smallerdiameter is disposed in one of said pairs of aligned openings and isterminated on one end substantially at the outer surface of the innerconductor with the shoulder in engagement with the inner conductor, andsuch that the portion of larger diame ter xtends from at least thedepressed perimeter to a substantially diametrically opposite portion ofthe inner surface of, the outer conductor.

3. A concentric conductor transmission line comprising tubular inner andouter conductors, said inner conductor having a plurality of pairs ofdiametrically aligned openings, one opening of each of said pairs havingits perimeter depressed below the outer surface of the inner conductor,and insulators to maintain concentric spacing between said inner andouter conductors, each insulator comprising two portions of differentdiameters and a shoulder formed at the junction of said two portions,each insulator being positioned-in one of said pairs of aligned openingssuch that the portion of smaller diameter 0):- tends substantiallybetween the outer surface of the depressed perimeter "and the outersurface of a substantially diametrically opposite portion of the innerconductor with the shoulder in engagement with the outer surface of thedepressed perimeter, and such that the portion of larger diameterextends from the shoulder to the inner surface of the outer conductor.

PHILLIP H. SMITH.

